In lithography, in order to produce semiconductor components by use of scanners or steppers, the structures of masks, which are also referred to as reticles, are projected onto wafers coated with a light-sensitive layer, the resist. Masks can be embodied for example as “binary masks” having chromium structures on quartz glass or as phase-shift masks. Reflective masks are used for application in EUV lithography. In mask inspection microscopes or position measuring devices, the structure of a reticle is projected onto a light-sensitive spatially resolved detector, such as a charge coupled device (CCD) chip, for example, with the aid of optical units.
By use of a positioning measuring device (registration tool), specific structure elements referred to as “registration pattern” or as “marker” on a mask, such as, for example, squares, crosses or angles having predefined shapes are measured and compared with the setpoint positions thereof. Positions of structure elements on the mask which are part of the used structures of the mask are also measured. This is referred to as “real pattern registration”. The deviation of the setpoint position of a structure element from the actual position thereof on the mask is the positioning error; the latter is also referred to as “registration” or “register error”.
The measurement of the masks makes it possible, in the mask writing process using electron beam writers, to check the positional accuracy of the structures on the mask. Furthermore, the measurement of the structures of an existing mask set makes it possible to qualify the deviation of the structure positions of the different masks for the individual lithographic layers with respect to one another.
For monitoring positions of structure elements, an aerial image of a segment of a mask is recorded by use of a position measuring device. The mask lies in this case on a stage (also referred to as specimen stage or displacing unit) which allows the mask to be displaced in the direction of the mask plane in order to enable the positioning of a desired segment in the image field of the position measuring device for recording the image by use of a detector. The mask is aligned before the measurement on the stage, such that its position on the stage is known. Alternatively, a relative alignment of the mask with respect to specific alignment structure elements can also be effected. The position determination is then effected relative to these structure elements, also referred to as alignment markers. Consequently, the image can be unambiguously assigned to the absolute or relative position of the segment on the mask. By determining the position of the structure within the recorded image, it becomes possible to compare setpoint and actual positions of the structures on the mask and thus to calculate the positioning error.
The requirements made of the accuracy of the measurement when determining positions of structure features on masks are 1 nm, but are to be improved to 0.5 nm and beyond in the next generation of devices.
When determining positions of structure elements, methods are used for determining the positions of said structure elements in aerial images. Methods such as, for example, threshold methods or correlation methods are known. The requirements made of the accuracy of the position to be determined or of the positioning error to be determined of a structure element are higher than the resolution of an aerial image recorded by the position measuring device.
DE 12006059431 describes a method for determining with subpixel accuracy the position of a structure on a carrier relative to a reference point of the carrier by superimposing the structure with a reference structure.
DE 10047211 discloses a method for determining the position of edges of a structure with subpixel accuracy. Here the measured edge profiles are compared with model intensity profiles in order to find the position of best correspondence.
DE 10337767 discloses a method for determining the distance between a reference element and a structure for measuring the overlay displacement.
DE 102007033815 discloses a method for determining the distance between a reference element and a structure with two pattern elements for measuring the overlay displacement, wherein one of the pattern elements, whose position is to be determined, is amplified in the reference element or in the structure.
The requirements made of the next generation of position measuring devices are still not attained by the application of these methods.